The invention relates to a method and an apparatus for regulation of image quality in a picture coding which is used in a picture/voice communication as wall as in a picture filing through communication networks such as GSTN, ISDN, PHS, PDC or the like, a picture communication equipment and also relates to a recording medium having a program recorded thereon which carries out the method in a computer.
According to ITU-T, H. 263, for example, five image formats are defined including SQCIF, QCIF, CIF, 4CIF and 16CIF. Of these, the image formats SQCIF, QCIF and CIF, which are illustrated by images F1, F2 and F3 shown in FIGS. 1A, 1B and 1C, respectively, are used for picture transmission/reception in T.V telephone or conference system. SQCIF comprises 128xc3x9796 picture elements, QCIF comprises 176xc3x97144 picture elements and CIF comprises 352xc3x97288 picture elements. An amount of data per picture element which is used to denote a gradation is determined by a picture quality setting, which is separately required and which in turn determines whether a motion in the image or the definition is to predominate, thus eventually determining a coding frame rate. Assuming that the picture quality should be motion oriented (which is equivalent to choosing a high coding frame rate), the amount of data per picture element is suppressed low to degrade the image quality while improving the frame rate. By contrast, if the picture quality should be image quality oriented (which is equivalent to reducing the coding frame rate), the amount of data per picture element is increased to improve the image quality while the frame rate is degraded. Thus it has been necessary in the prior art for a user interface which regulates the image quality in the TV-telephone or conference system to provide a combination of both the image format and the picture quality of a picture being transmitted.
FIG. 2 illustrates an exemplary user interface which occurs on a conventional computer display screen. A user may depress either selection button D22 on a user interface screen D20 to change the image format or the picture quality. The term xe2x80x9cpicture qualityxe2x80x9d is used herein for the ease of understanding by a general reader, but actually refers to a coding frame rate. An active sign D21 allows an indication of which one of the image format and the picture quality is to be changed. By using the selection button D22 to cause the active sign D21 to be activated for the indication xe2x80x9cimage formatxe2x80x9d and by depressing either change button D23, a change in the image format is enabled. In response to the depression of the change button D23, a change to a desired image format such as CIF, QCIF, SQCIF or the like takes place, as shown by a format display D24. Similarly, an active sign may be activated for the picture quality by using the selection button D22, and the change button D23 may then be depressed to change the picture quality (or the coding frame rate). The depression of the change button D23 is effective to choose either xe2x80x9cmotion orientedxe2x80x9d, as indicated by a corresponding display D25, or xe2x80x9cimage quality orientedxe2x80x9d.
FIG. 3 shows a procedure of operating the conventional user interface. When a communication is initiated and it is determined at step S1 that it is desired to change the image format, such change takes place at step S2. When it is determined at step S3 that it is desired to change the picture quality (coding frame rate), such change takes place at step S4. When it is determined at step S5 that a total satisfactory picture quality as desired by a user has been achieved, the operation proceeds to a following processing procedure. Otherwise, the operation returns to a decision at step S1. Thus the regulation of the image quality in the user interface requires repeated procedures for combining the image format and the picture quality of a picture being transmitted.
A method of setting up an image quality to provide an animated picture so that a speaker in the TV-telephone appears to be behaving in a natural manner is known in the art, as disclosed in Japanese Laid-Open Patent Application No. 328, 341/1993, where coding parameters had to be changed depending on whether the amount of compacted data is greater or less than a given value. This technique is similar to a coding quantity control which is premised for a variety of picture coding schemes (H.261, H.262, H.263) which are chosen under consideration by the international standards organization ITU-T, and in its present form, requires at least two parameters, the image format and the frame rate, to be changed when a user regulates the image quality.
Thus it will be seen that with the prior art approach, before initiating a communication, a user has to operate the two parameter, the image format and the picture quality, in his user interface of a picture communication equipment in order to obtain the quality and the motion of the picture as desired by the user. At this end, in setting up the regulation of the total image quality for the picture, the user has to choose one of image qualities corresponding to values Q11, Q21, Q31, Q12, Q22 and Q32 as shown by a chart in FIG. 4 from combinations of the two parameters, the image format and the picture quality, requiring repeated procedures which demand a considerable amount of time and labor at the initiation of communication.
It is an object of the invention to provide a method of regulation of a total image quality in a coded picture which realizes the total image quality regulation through a single operation without requiring an independent operation of an image format and a frame rate, a picture communication equipment using the method, and a recording medium having recorded therein a program for executing the method.
According to a first aspect of the invention, a method of regulating an image quality of a picture by regulating coding parameters when the picture is coded in response to image quality regulating parameters comprises the steps of
(a) previously storing characteristic information which defines an applicable range of coding frame rates for each image format in response to at least one, predetermined coding bit rate;
(b) inputting a parameter which specifies an image quality; and
(c) determining the image format and the coding frame rate by reference to the characteristic information using the parameter as a key.
According to a second aspect of the invention, there is provided a method of regulating a picture coding according to the method according to the first aspect of the invention which comprises the steps of determining a coding time required to encode each image format, calculating a number of picture frames which can be used for each of the image formats on the basis of a result of the determination, and determining a picture format and a coding frame rate on the basis of a result of the calculation and image quality regulating parameters which are input.